Light

The non amateur stuff. Hawking, black holes, that sort of thing

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Moonstruck
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Light

Post by Moonstruck »

delette
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stella
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Post by stella »

You seem to be using "light years" as a measure of time, but it is a unit of distance.
Moonstruck
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Post by Moonstruck »

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joe
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Post by joe »

Hi Moonstruck,

A light year is purely a measure of distance although the "year" part often causes confusion. It is of course equal to the distance light travels in a year which is 9460000000000 kms.

As far as I know light doesn't decay but the wavelength of light can be "stretched" due to the expansion of the universe. For instance the Cosmic Micro-wave Background Radiation was initialy emitted as visible light (wavelength ~ 0.00007cms) over 13 billion years ago but as the universe expanded, the wavelength of this light has been stretched into the micro-wave region (wavelength ~ 1cm). Still stretching I suppose!
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Cliff
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Post by Cliff »

Dear Moonstruck
I never like agreeing with anybody, but I have to agree with Stella and Joe on this one. Like Joe as far as I know light does not fade (although it might depend on what you actually mean by the word fade). I have read at times that I think it was in the 1940s Fritz Zwicky suggested the posibilty of there being "tired light". From what I have read about Fritz there is no way I would argue with him, argumentative s-d though I might be. As far as I know the tired light idea never got anywhere, perhaps it was too tired!
Best wishes from the Grumpy old Codger Cliff
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Post by joe »

Cliff wrote:From what I have read about Fritz there is no way I would argue with him,
I've read in quite a few books about the odd behaviour of Zwicky but never any real examples. Anybody know enough to spill the beans?
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Moonstruck
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Post by Moonstruck »

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davep
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Post by davep »

joe wrote:Anybody know enough to spill the beans?
His Wikipedia entry seems to give a bit of a taste of what he was like. I really love the phrase it reports him as using to refer to people who disagreed with him.
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Re: Light

Post by joe »

Moonstruck wrote: I'm still confused over the explainations.
Light traveling at that very long number in a very short space of time; s*d=time. Does it?
Sorry Moonstruck, I'm not 100% sure of what it is exactly you are asking. Light coming from a star in a distant galaxy reaches our eyes, or telescope, because it wasn't "blocked" by anything else on that journey, obviously. It would have continued to travel if it hadn't been received by our telescope and indeed most of the photons (light particles) do continue their journey outwards from the star. The photons do not fade but become more and more spread out because the same amount of photons occupies a bigger volume of space. There may have been one trillion photons per cubic centimetre at the surface of the star for example (although probably rather more) but by the time it reached our eyes there might be only a few hundred photons per cubic centimetre. Each photon will carry on indefinitely until it hits or is absorbed by something else. A photon may take billions of years to reach us but that is no time at all in the "life" of a photon. Its wavelength would be stretched, as I said before, due to universal expansion and would make it less energetic but it would still carry on regardless as micro-waves or radio waves which as you know are not detectable by the human eye.

Is this of any help or am I telling you lots of stuff you know already?
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Post by Cliff »

Dear Joe
Fritz Zwicky is one of my many heroes and he may not have been a very likeable chap really. Although to be honest I do not really know very much about him. A couple of things I have read, I think he was a wizard on Supernovae and earlish days of Black Holes. Apparently Walter Baade who was a great astronomer (I think Baade was very involved in better understanding Cepheid variables resulting in M31 distance effectively doubling more or less overnight) and collaborator with Fritz, was supposedly scared to death of Zwicky. Even though they were supposed to be collaborators, I read that Baade would not stay in the same room as Zwicky unless there was someone else present as well.
Apparently Zwicky regularly called poor old Baade a NAZI which was very much the opposite to reality. I must confess I personally would have liked Zwicky to have sorted out Werner Von Brawn, but I suspect Von Brawn could probably have looked after himself better than poor old Walter Baade. But that may be wishfull thinking on my part.
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Post by Cliff »

Dear Joe
Fritz Zwicky is one of my many heroes and he may not have been a very likeable chap really. Although to be honest I do not really know very much about him. A couple of things I have read, I think he was a wizard on Supernovae and earlish days of Black Holes. Apparently Walter Baade who was a great astronomer (I think Baade was very involved in better understanding Cepheid variables resulting in M31 distance effectively doubling more or less overnight) and collaborator with Fritz, was supposedly scared to death of Zwicky. Even though they were supposed to be collaborators, I read that Baade would not stay in the same room as Zwicky unless there was someone else present as well.
Apparently Zwicky regularly called poor old Baade a NAZI which was very much the opposite to reality. I must confess I personally would have liked Zwicky to have sorted out Werner Von Brawn, but I suspect Von Brawn could probably have looked after himself better than poor old Walter Baade. But that may be wishfull thinking on my part.
Best wishes from the Grumpy Old Codger Cliff
Moonstruck
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Post by Moonstruck »

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davep
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Re: light

Post by davep »

Moonstruck wrote:After all light traveling at a bat of an eyelid still takes a measurable time
While that is true a light year is a measure of distance, not of time. You can't measure time with a light year because time is what defines a light year.
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Re: light

Post by joe »

Moonstruck wrote:Speed*Distance=Time a light year?
Speed = Distance/Time

The nearest star to us is Proxima Centauri which is 42570000000000kms away but as this distance is so ridiculously far, small units like kilometres are not really suitable. A bigger unit is needed and the distance light travels in a year is a large unit as is explained above, called a lightyear. Dividing the distance to Proxima Centauri 42570000000000km by a lightyear 9460000000000km gives a distance of 4.5 lightyears, a much more manageable unit. The fact that the unit lightyear originally involved distance travelled in a year, a measure of time, is irrelevent to the unit, a lightyear is simply a distance. A metre is a unit of distance, no difficulty there but in fact a metre is defined by the distance light travels in 1/299792558 of a second.

You could make up your own units. MOONSTRUCK HOURS for example, which would be the distance you walk in an hour. What's that then... half a mile? :wink: Therefore Roger Banister ran the "2 Moonstruck Hours" in under four minutes.
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stella
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Post by stella »

Nice example Joe,
One that I often quote is the Kilowatt hour which is not a measure of time either.
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